FIG. 1 illustrates schematically in cross-section a mobile lens unit 100 comprising a cylindrical lens barrel 102 with a number of fixed lenses 104, 106 and 108 mounted therein, the lens barrel 102 having a conducting coil 110 mounted within its walls. The coil 110 is formed of a wire represented in cross-section by small circles with dots to represent a current flow one way in the wire, and crosses to represent a current flow in the opposite direction. Mobile lens unit 100 also comprises a housing 112, which is for example formed of plastic, and has rims 113, 114 at the top and bottom respectively, which extend some way towards the center of the housing, and act as stoppers for the lens barrel 102. A permanent magnet 115, which is for example also cylindrical, is mounted against the inner surface of the housing 112 and lies adjacent to the lens barrel 102. The coil 110, and the permanent magnet 115 together form a voice coil motor (VCM). Springs 116 and 118 are connected between the housing 112 and the lens barrel 102.
Coil 110 is connected to a current source, in this example by conductive springs 116 and 118. In particular, springs 116 and 118 connect opposite ends of coil to a control circuit 120, which comprises a variable current source coupled between spring 116 and ground, and a supply voltage Vdd connected to spring 118.
In operation, the mobile lens unit 100 is mounted over an image sensor (not shown), and the positioning of the lenses with respect to the image sensor can be adjusted by moving the lens barrel 102 up and down within the housing, which can be achieved by passing a current signal through the coil 110. Springs 116, 118 hold the lens barrel 102 at an first position at the bottom end of the housing resting against rim 113 when no current is applied to the coil. When current is applied to the coil, due to electromagnetic force generated by the current flow in a magnetic field, the lens barrel moves towards the top of the housing, counteracting the force of the springs 116, 118. Springs 116, 118 have an increasing restoring force the further the lens barrel is from its first position, meaning that the lens barrel rests at in equilibrium at a certain position depending on the current level applied to the coil.
FIG. 2A illustrates the current I applied to the coil for changing the position of the lens barrel 102 in the housing 112. As illustrated, the current is initially at an value Ii, which maintains the lens barrel at the first position. The first position could be with the lens barrel 102 at the bottom of the housing 112, or at any position within the lens housing. At a time t0, a new current If is applied, which has a value corresponding to the current that is needed to keep the lens barrel at the final position.
FIG. 2B illustrates the position x of the lens barrel in response to the change in the current at time t0. As illustrated, from the initial position xi, the lens barrel moves towards the final position xf. However, the lens barrel 102 and springs 116, 118 form a mass spring system that oscillates before settling at the final position xf. Damping in the system is very low, as the lens barrel is designed with very little friction, in order to reduce energy consumption. Thus the oscillations can continue for some time before the lens barrel settles at the final position. These oscillations cause problems, as the mobile lens unit cannot be used to capture images until the oscillations of the lens barrel have settled to within a certain amplitude, below which they do not affect the focusing of the lens. This settling time depends on the decay rate of the system, and can be several hundred milliseconds, meaning that there is a considerable delay before the lens can be used. In many optical applications, this delay is unacceptable.
This problem occurs when a lens barrel is moved upwards in the lens unit, by a step increase in the current, and also when the lens barrel is moved downwards in the lens unit, by a step decrease in the current. A further problem that can occur when the lens barrel 102 is to be moved to a final position close to the end stops provided by rims 113, 114 is that the oscillations cause an impact between the lens barrel and the end stops, which could damage the lens barrel and create unwanted noise.
One solution to these problems that has been proposed is to slowly change the current in the coil from the initial current to the final current. While this can help reduce the magnitude of oscillations, there is still a considerable delay before the lens unit can be used, particularly in smaller lens barrels designed for miniature application such as mobile telephones in which the friction of the lens barrel is extremely low.